In the practice of many industrial and chemical processes, hazardous environments may be present. For instance in ammunition plants, coal mines, grain processing plants, and chemical plants, hazardous environments can exist such that if a wire fault or other electrical abnormality creates a spark, or if the temperature of electrical devices operating in the hazardous environments becomes too high, an explosive condition can occur. Therefore when electrical components, such as sensing systems for detecting the presence of an object, are utilized, it is necessary to limit the electrical energy to and temperature of the sensing components within the hazardous environments.
Conventional techniques for limiting electrical energy to, and the temperature of, electrical components within a hazardous environment (such as photoelectric sensing mechanisms) commonly include the use of a separate energy limiting safety barrier in series with each signal lead to the hazardous environment (see American National Institute Report RP 12.6, Installation of Intrinsically Safe Instrument Systems in Class 1 Hazardous Locations). Such techniques have very reduced sensitivity of the photoelectric sensors or the like used in the hazardous environment, and are expensive due to the addition of components and assembly time. Another technique commonly used (see National Electrical Code of the National Fire Protection Association, 1978, Chapter 5) is the use of explosion-proof wiring to each sensor. This technique is very expensive and for that reason undesirable in many situations. Thus, there is a need for an intrinsically safe sensing system (and simple component parts therefor) which effect energy and temperature limitation without degradation of the sensor return signal, and which provides for safety and in simple, inexpensive manner.
According to the present invention, this need for an intrinsically safe sensing system which does not degrade the return signal from the sensor, and which provides its energy and temperature limiting functions in a simple and inexpensive manner, is satisfied. A sensing system according to the present invention includes a photoelectric sensing means including a photoelectric transmitter and a photoelectric receiver. Circuitry means operatively interconnect the photoelectric sensing means to a power source, the circuitry means including electrical barrier means for limiting the energy to the sensing means to a safe level. An electrical cable extends between the photoelectric sensing means and the circuitry means, the cable being long enough to provide for positioning of the sensing means in the hazardous area and positioning of the circuitry means in the safe area. The circuitry means are provided in a single module located in the safe area.
The circuitry means comprises a transmitting circuit with barrier means, and a receiving circuit with barrier means. The transmitting circuit includes a transistor electrically interconnected between the cable and the transmitting circuit barrier means, and the receiving circuit includes a preamplifier connected between the cable and the receiver circuit barrier means, so that a signal from the photoelectric receiver will pass through the barrier means without significant degradation. The circuitry means further comprises means for supplying a low frequency ac current to the photoelectric transmitter and for receiving the low frequency ac current from the photoelectric receiver, the photoelectric transmitter and receiver being ac coupled.
The present invention is ideally suited for use with photoelectric sensing means, although various aspects of the invention may be practiced in conjunction with other types of sensing means. One feature particularly related to the cost and size reduction goals desirably achieved according to the invention is the utilization of a single module mounting the circuitry means. The module includes a base having a safe area end and a hazardous area end, with a first set of terminals on the safe area end of the base and a second set of terminals on the hazardous area end. The transmitting and receiving circuits are mounted on the base electrically connected between the first and second terminal sets, and means are provided for encapsulating the transmitting and receiving circuits to prevent access thereto, while allowing testing thereof. Signal processing circuitry, an output signal activated relay, and an oscillator and phase detector are also mounted on the base, the module thus comprising the only component that need be provided between the power supply and the electrical cable extending to the sensor within the hazardous area.
It is the primary object of the present invention to provide a simple, efficient, and cost-effective intrinsically safe sensing system, and components therefor. This and objects of the present invention will become clear from an inspection of the detailed description of the invention and from the appended claims.